The subject-matter of the present invention is a process for the preparation of azine and a process for the preparation of hydrazine employing it.
The synthesis of hydrazine from ammonia and hydrogen peroxide is described in Ullmann""s Encyclopedia of Industrial Chemistry (1989), Vol. A 13, pages 182-183.
In a first stage, ammonia, hydrogen peroxide and a reactant carrying a carbonyl group are reacted in a reactor in order to form an azine, according to the reaction (I): 
In this scheme, R1 and R2, which are identical or different, represent a hydrogen atom or a C1-C4 alkyl radical, provided that at least one of R1 and R2 is other than a hydrogen atom, or else R1 and R2 form, together with the carbon atom to which they are bonded, a C3-C6 cycloalkyl radical.
This reaction is necessarily carried out in the presence of a catalyst (or activator) or of a mixture of catalysts included within a composition denoted as working solution. At the end of the reaction, the azine is separated from the working solution. The latter is then regenerated and then recycled in the reactor of Stage (I).
In a second stage, the azine is hydrolysed to hydrazine, according to the reaction (II): 
the reactant carrying a carbonyl group is recovered and recycled in the reactor of Stage (I).
This process was disclosed in particular in U.S. Pat. Nos. 3,972,878 and 3,972,876.
The term xe2x80x9cworking solutionxe2x80x9d is therefore understood to mean any aqueous solution or suspension comprising a catalyst or a mixture of catalysts which is capable of converting a mixture of ammonia, of hydrogen peroxide and of a reactant carrying a carbonyl group to an azine.
This working solution is disclosed in Patents EP 399,866, EP 487,160 and EP 70,155. It is composed, for example, of an aqueous solution of acetamide and of ammonium acetate. The ammonium acetate is formed in the reactor, in particular by reaction of acetic acid with ammonia. The working solution can also be composed of an aqueous solution of cacodylic acid and of ammonium cacodylate.
Application EP 0,518,728 discloses a process for the synthesis of azine in which the working solution and more generally any stream entering the reactor for the synthesis of the azine are devoid of CO2. According to this application, the presence of CO2 in the synthesis process generally results from undesirable reactions of the hydrogen peroxide with various organic impurities which are produced in certain stages of the process. The CO2 thus formed reacts in its turn with the hydrogen peroxide, which results in an excessive consumption of H2O2 and a fall in the yield of azine intermediate and thus of hydrazine.
To overcome this disadvantage, Application EP 0,518,728 discloses a process for the preparation of intermediate azine, in which
a) ammonia, hydrogen peroxide and a reactant carrying a carbonyl group are brought into contact in a reactor with a working solution, then
b) the azine thus formed is separated from the working solution, then
c) the working solution is regenerated by bringing it to a temperature of at least 130xc2x0 C., so that the CO2 and most of the water (formed during the reaction of Stage a) or contributed by the water of dilution of the hydrogen peroxide) are removed in the form of an aqueous stream, denoted C1 in the continuation of the present text, which therefore comprises, in the dissolved state, in addition to the CO2 (essentially present in the carbonate form), a certain amount of the ammonia and of the reactant carrying a carbonyl group employed in Stage a), then
d) the working solution, regenerated according to Stage c), is recycled in the reactor of Stage a).
The stream C1 comprises a certain amount of ammonia and of reactant carrying a carbonyl group. However, it cannot be directly recycled in the reactor for the synthesis of the azine, specifically because of the presence of a significant amount of CO2. In order to economize on the significant amounts of reactants which can be used in the process, Application EP 0,518,728 therefore recommends complementing Stage d) of the said process by treating this stream C1 so as to remove the CO2 therefrom, in order to be able thus to recycle the ammonia and the reactant carrying a carbonyl group which it comprises in the reactor for the synthesis of azine. FIG. 1/3 represents a device suited to the implementation of this process. It is described in a detailed way later in the present text, in the appropriate part.
A process for the treatment of the stream C1 appropriate for the removal of CO2, such as the removal mentioned in Stage d), is also disclosed by Application EP 0,518,728.
According to this process, the stream C1 is first of all treated in a stripping column, the top stream of which is cooled in a condenser. The NH3 is collected in the gaseous state, and the liquid phase obtained is separated, in a settling tank, into the organic phase, rich in reactant carrying a carbonyl group, and the aqueous phase.
The NH3 and the organic phase thus obtained are recycled to the reactor for the synthesis of the azine.
The aqueous phase, which comprises all the CO2 present in the stream C1, is fed to a 2nd distillation column, so as to collect:
at the column top, an aqueous stream devoid of CO2 and comprising a certain amount of ammonia and of reactant carrying a carbonyl group, which is therefore recycled to the reactor for the synthesis of the azine,
at the column bottom, an aqueous stream, denoted C2 in the continuation of the present text, which comprises virtually all the CO2 present in C1 and also small amounts of ammonia and of reactant carrying a carbonyl group.
FIG. 2/3 represents, as a reminder, the device taught by EP 518, 728 for the treatment of the stream C1.
Under the practical conditions of implementation of the process, the Stream C2 generally comprises from 0.1 to 20% by weight of CO2 (present in the form of carbonate in the dissolved state), from 1 to 15% of NH3 and from 0.1 to 10% of reactant carrying a carbonyl group.
In the present text, except when otherwise indicated, the percentages shown for the components are percentages expressed by weight.
Application EP 0,518,728 gives no teaching relating to the treatment to be given to the Stream C2. According to this application, it cannot be recycled to the reactor for the synthesis of the azine, since it comprises CO2.
The removal of such a Stream C2 might be envisaged by means of known techniques, such as incineration. However, this removal would necessarily be accompanied by releases to the atmosphere of a mixture of compounds of the NO or NO2 type, resulting from the combustion of NH3. In point of fact, it is desirable to limit as far as possible the discharge to the atmosphere of such compounds. Furthermore, this removal would also involve the loss of small amounts of reactants which can be used in the process, namely ammonia and reactant carrying a carbonyl group, which loss is harmful to the economics of the process.
Application EP 0,518,728 suggests removing the CO2 by washing with sodium hydroxide solution, in order to retain the CO2 in the form of an aqueous sodium carbonate solution and to leave the ammonia, the reactant carrying a carbonyl group and water vapour in gaseous form.
A novel process for the preparation of the azine has now surprisingly been found which, in contrast to the teaching of Patent EP 0,518,728, involves an acid in the treatment of the Stream C2.
One aim of the invention is therefore to provide a process for the preparation of azine in which the discharges of effluents to the environment are reduced.
Another aim of the invention is to provide a process for the preparation of azine in which the losses of reactants of use in the reaction are reduced.
These aims can be achieved, in all or in part, by the process for the preparation of azine which is the subject-matter of the invention, which process is described hereinbelow.
The subject-matter of the invention is therefore, firstly, a process for the preparation of azine comprising the stages:
(i) in which, successively
(a) ammonia, hydrogen peroxide and a reactant carrying a carbonyl group are brought into contact, in a reactor, with a working solution,
(b) the azine thus formed is separated from the working solution,
(c) the working solution is regenerated by bringing it to a temperature of at least 130xc2x0 C., so that the CO2 and most of the water (formed during the reaction of Stage a) or contributed by the water of dilution of the hydrogen peroxide) are removed in the form of an aqueous stream C1, which therefore comprises, in the dissolved state, in addition to the CO2 (present essentially in the carbonate form), a certain amount of ammonia and of the reactant carrying a carbonyl group employed in Stage a),
(d) the working solution, regenerated according to Stage c), is recycled in the reactor of Stage a); and
(ii) in which, successively
(a) the said stream C1 is treated in a stripping column;
(b) the top stream is condensed while separating the NH3 in the gaseous state;
(c) the liquid phase resulting from the condensation is separated into an organic phase, rich in reactant carrying a carbonyl group, and an aqueous phase comprising all the CO2 present in C1;
(d) the said aqueous phase is fed into a distillation column, from where an aqueous stream devoid of CO2 is collected at the top and an aqueous stream C2 comprising virtually all the CO2 present in C1 is collected at the bottom,
characterized in that
at least a portion of the stream C2 is treated with an amount of acid such that the pH of the said stream is brought to a value of less than 6.4, preferably less than 6 ; then
the said portion (denoted stream C3) thus treated is recycled in the reactor of Stage (i)(a).
This process is particularly advantageous in that it makes it possible to recover most of the amounts of NH3 and of reactant carrying a carbonyl group present in the stream C1, in order for them to be reused in the reactor for the synthesis of the azine. This results in an appreciable saving. Furthermore, in contrast to the treatment with sodium hydroxide solution suggested by EP 0,518,728, it makes it possible to avoid any problem of liquid or solid discharge to the environment, the CO2 being removed by simple degassing.
The treatment with the acid is usually carried out at a temperature of between 20 and 70xc2x0 C., preferably between 40 and 60xc2x0 C., and at a pressure of between 0.5 and 4 bar absolute, preferably between 0.5 and 2 bar absolute.
The aqueous stream C2 employed usually has a CO2 content of between 0.1 and 20%, preferably between 0.5 and 5%, an NH3 content of between 1 and 15%, preferably between 1 and 6%, and a content of reactant carrying a carbonyl group of between 0.1 and 10%, preferably between 0.1 and 5%. The pH of the aqueous stream C2 is usually between 8 and 12.
It is preferable to subject the whole of the Stream C2 to the treatment with the acid.
It is also preferable to employ the process according to the invention while choosing methyl ethyl ketone as reactant carrying a carbonyl group. In this case, the corresponding azine is insoluble in aqueous solution, which facilitates Stage (i)(b) of separation of the azine from the working solution.
The acid employed in the treatment of the aqueous stream C2 is advantageously an acid with a pKa of less than 6.3, preferably of less than 5.
According to a preferred alternative form of the process according to the invention, the working solution employed comprises an aqueous solution of acetamide and of acetic acid. In this case, it is preferable to use acetic acid in the treatment of the Stream C2, as acid with a pKa of less than 6.3.
According to another preferred alternative form of the process according to the invention, the working solution employed comprises an aqueous solution of cacodylic acid and of ammonium cacodylate. In this case, it will be preferable to use cacodylic acid for the treatment of the Stream C2, as acid with a pKa of less than 6.3.
In all cases, the treatment by neutralization of the Stream C2 can be carried out in a very simple, more particularly preferred, way by use of the stream of acid introduced into the circuit for the regeneration of the working solution in order to compensate for the losses occasioned by the circulation and treatment of the solution. The treatment of the Stream C2, which is carried out using a reactant already employed in the process, is consequently very advantageous in economic terms.
Another subject-matter of the invention is a process for the preparation of hydrazine, comprising the process for the preparation of the azine as described above, in which the azine separated from the working solution in accordance with Stage (i) (b) of the said process is hydrolysed to hydrazine, the reactant carrying a carbonyl group being recovered and recycled in the reactor for the synthesis of the azine.